Scatter Air Ratio

SARs are used for the purpose of calculating scattered dose in the medium. The computation of the primary and the scattered dose separately is particularly useful in the dosimetry of irregular fields.

SAR may be defied as the ratio of the scattered dose at a given point in the phantom to the dose in free space at the same point. The SAR, like the TAR, is independent of the SSD but depends on the beam energy, depth, and field size.
Because the scattered dose at a point in the phantom is equal to the total dose minus the primary dose at that point, SAR is mathematically given by the difference between the TAR for the given field and the TAR for the 0 × 0 field

SAR (d, rd) = TAR (d, rd) - TAR (d,0)

Here TAR (d,0) represents the primary component of the beam.

Khan's The Physics of Radiation Therapy Fifth Edition

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TAR or Tissue Air Ratio

Tissue-Air Ratio

Tissue-air ratio (TAR) was first introduced by Johns  in 1953 and was originally called the “tumor-air ratio.” At that time, this quantity was intended specifically for rotation therapy calculations. In rotation therapy, the radiation source moves in a circle around the axis of rotation, which is usually placed in the tumor. Although the SSD may vary depending on the shape of the surface contour, the source-axis distance remains constant.
Since the percent depth dose depends on the SSD , the SSD correction to the percent depth dose will have to be applied to correct for the varying SSD—a procedure that becomes cumbersome to apply routinely in clinical practice. A simpler quantity—namely TAR—has been defined to remove the SSD dependence. Since the time of its introduction, the concept of TAR has been refined to facilitate calculations not only for rotation therapy, but also for stationary isocentric techniques as well as irregular fields.

Tissue-air ratio may be defined as the ratio of the dose (D_d) at a given point in the phantom to the dose in free space (D_fs) at the same point.

This is illustrated in Figure 2 For a given quality beam, TAR depends on depth d and field size r_d at that depth:

Fig 2: Illustration of the definition of tissue-air ratio (TAR). TAR(d,r_d) = D_d/D_fs

Physics of Radiation Therapy, The, 5th Edition

Faiz M. Khan PhD

Professor Emeritus

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